Apparatus for analyzing multi-layer thin film stacks on semiconductors

ABSTRACT

An optical measurement system is disclosed for evaluating samples with multi-layer thin film stacks. The optical measurement system includes a reference ellipsometer and one or more non-contact optical measurement devices. The reference ellipsometer is used to calibrate the other optical measurement devices. Once calibration is completed, the system can be used to analyze multi-layer thin film stacks. In particular, the reference ellipsometer provides a measurement which can be used to determine the total optical thickness of the stack. Using that information coupled with the measurements made by the other optical measurement devices, more accurate information about individual layers can be obtained.

This application is a continuation of prior U.S. application Ser. No.09/563,152, filed May 2, 2000, now U.S. Pat. No. 6,297,880, which inturn is a continuation of U.S. application Ser. No. 09/015,839 filedJan. 29, 1998, now U.S. Pat. No. 6,278,519.

FIELD OF THE INVENTION

The present invention relates to optical analyzers, and moreparticularly to an optical measurement system having a stable singlewavelength ellipsometer and a broadband spectroscopic measurement moduleto accurately characterize multi-layer thin film stacks.

BACKGROUND OF THE INVENTION

There is considerable interest in developing systems for accuratelymeasuring the thickness and/or composition of multi-layer thin films.The need is particularly acute in the semiconductor manufacturingindustry where the thickness of these thin film oxide layers onsemiconductor substrates is measured. To be useful, the measurementsystem must be able to determine the thickness and/or composition offilms with a high degree of accuracy. The preferred measurement systemsrely on non-contact, optical measurement techniques, which can beperformed during the semiconductor manufacturing process withoutdamaging the wafer sample. Such optical measurement techniques includedirecting a probe beam to the sample, and measuring one or more opticalparameters of the reflected probe beam.

In order to increase measurement accuracy and to gain additionalinformation about the target sample, multiple optical measuring devicesare often incorporated into a single composite optical measurementsystem. For example, the present assignee has marketed a product calledOPTI-PROBE, which incorporates several optical measurement devices,including a Beam Profile Reflectometer (BPR), a Beam ProfileEllipsometer (BPE), and a Broadband Reflective Spectrometer (BRS). Eachof these devices measures parameters of optical beams reflected by thetarget sample. The BPR and BPE devices utilize technology described inU.S. Pat. Nos. 4,999,014 and 5,181,080 respectively, which areincorporated herein by reference.

The composite measurement system mentioned above combines the measuredresults of each of the measurement devices to precisely derive thethickness and composition of the thin film and substrate of the targetsample. However, the accuracy of the measured results depends uponprecise initial and periodic calibration of the measurement devices inthe optical measurement system. Further, recently developed measurementdevices have increased sensitivity to more accurately measure thinnerfilms and provide additional information about film and substratecomposition. These newer systems require very accurate initialcalibration. Further, heat, contamination, optical damage, alignment,etc., that can occur over time in optical measurement devices, affectthe accuracy of the measured results. Therefore, periodic calibration isnecessary to maintain the accuracy of the composite optical measurementsystem.

It is known to calibrate optical measurement devices by providing areference sample having a known substrate, with a thin film thereonhaving a known composition and thickness. The reference sample is placedin the measurement system, and each optical measurement device measuresthe optical parameters of the reference sample, and is calibrated usingthe results from the reference sample and comparing them to the knownfilm thickness and composition. A common reference sample is a “nativeoxide” reference sample, which is a silicon substrate with an oxidelayer formed thereon having a known thickness (about 20 angstroms).After fabrication, the reference sample is kept in a non-oxygenenvironment to minimize any further oxidation and contamination thatchanges the thickness of the reference sample film away from the knownthickness, and thus reduces the effectiveness of the reference samplefor accurate calibration. The same reference sample can be reused toperiodically calibrate the measurement system. However, if and when theamount of oxidation or contamination of the reference sample changes thefilm thickness significantly from the known thickness, the referencesample must be discarded.

For many optical measurement devices, reference samples with knownthicknesses have been effective for system calibration. Oxidation andcontamination that routinely occurs over time with reference samples istolerable because the film thickness change resulting from theoxidation/contamination is relatively insignificant compared to theoverall thickness of the film (around 100 angstroms). However, newultra-sensitive optical measurement systems have been recently developedthat can measure film layers with thicknesses less than 10 angstroms.These systems require reference samples having film thicknesses on theorder of 20 angstroms for accurate calibration. For such thin filmreference samples, however, the changes in film layer thicknessresulting from even minimal oxidation or contamination are significantcompared to the overall “known” film layer thickness, and result insignificant calibration error. Therefore, it is extremely difficult, ifnot impossible, to provide a native oxide reference sample with a knownthickness that is stable enough over time to be used for periodiccalibration of ultra-sensitive optical measurement systems.

There is a need for a calibration method for ultra-sensitive opticalmeasurement devices that can utilize a reference sample that does nothave a stable or known film thickness.

There is also a need in the industry to improve the accuracy of thesetype of measuring systems to permit characterization of samples havingmultiple thin film layers formed thereon. More particularly, in thesemiconductor industry, semiconductor material substrates are now beingfabricated with multiple thin film layers. Each film layer can be formedfrom a different material. Common layer materials include oxides,nitrides, polysilicon, titanium and titanium-nitride.

Attempts to characterize samples having multiple thin layers withconventional techniques is difficult since each layer has a differentthickness and different optical characteristics. The best approachesfound to date to characterize such complex stacks is to utilize multiplemeasurement techniques which generate independent data that can beanalyzed by a processor. Devices now exist which are capable of makingboth ellipsometric (phase) and spectrophotometric (magnitude)measurements and integrating the results in a microprocessor. Theellipsometers in these devices can include multiple wavelength andmultiple angle of incidence measurements. Similarly, thespectrophotometers in some of these devices can be arranged to makemeasurements at multiple angles of incidence.

While these systems have had reasonable success, further accuracy inanalyzing the characteristics of individual layers in a multi-layerstack is always desirable. The subject system, which includes awavelength stable calibration ellipsometer can be modified to improvethe characterization of individual layers of multi-layer thin filmstack.

SUMMARY OF THE INVENTION

The present invention is a thin film optical measurement system with awavelength stable ellipsometer that can be used for calibration and toenhance the characterization of multi-layer thin film stacks. When usedfor calibration purposes, the stable wavelength ellipsometer functionsto precisely determine the thickness of a film on a reference sample.The measured results from the calibration ellipsometer are used tocalibrate other optical measurement devices in the thin film opticalmeasurement system. By not having to supply a reference sample with apredetermined known film thickness, a reference sample having a filmwith a known composition can be repeatedly used to calibrateultra-sensitive optical measurement devices, even if oxidation orcontamination of the reference sample changes the thickness of the filmover time.

The calibration reference ellipsometer uses a reference sample that hasat least a partially known composition to calibrate at least one othernon-contact optical measurement device. The reference ellipsometerincludes a light generator that generates a quasi-monochromatic beam oflight having a known wavelength and a known polarization for interactingwith the reference sample. The beam is directed at a non-normal angle ofincidence relative to the reference sample to interact with thereference sample. An analyzer creates interference between S and Ppolarized components in the light beam after the light beam hasinteracted with reference sample. A detector measures the intensity ofthe light after the beam has passed through the analyzer. A processordetermines the polarization state of the light beam entering theanalyzer from the intensity measured by the detector. The processor thendetermines optical properties of the reference sample based upon thedetermined polarization state, the known wavelength of light from thelight generator and the at least partially known composition of thereference sample. The processor operates at least one other non-contactoptical measurement device that measures an optical parameter of thereference sample. The processor calibrates the other optical measurementdevice by comparing the measured optical parameter from the otheroptical measurement device to the determined optical property from thereference ellipsometer.

The reference ellipsometer has the further benefit in that it can beused to very accurately measure the overall optical thickness of anunknown multi-layer stack on a substrate. In this context, the termtotal optical thickness refers to the effective thickness of the stackwhich corresponds to a single uniform layer with uniform opticalparameters (i.e. n and k). A stable wavelength ellipsometer is anexcellent tool for determining the total optical thickness of a layer ora stack having a thicknesses less than 500 angstroms and is the besttool for stacks having a thickness of 200 angstroms or less.

The reference ellipsometer, which provides only a single wavelength,single angle of incidence output, is not suitable for analyzing theindividual layers in a stack. Such analysis requires additionalmeasurements typically from spectroscopic tools such asspectrophotometers and spectroscopic ellipsometers. However, the lattertools alone have difficulty producing sufficient information toaccurately characterize the stack.

In accordance with the subject invention, the output from the wavelengthstable ellipsometer is used by the processor to determine the overalloptical thickness of the multi-layer stack. This information is used bythe processor to reduce the uncertainty of the analysis based on thespectroscopic measurements. By taking a number of measurements atdifferent wavelengths with one or more different techniques, veryaccurate information about layer composition and thickness can bedetermined.

Other aspects and features of the present invention will become apparentby a review of the specification, claims and appended figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a composite optical measurement system with thecalibration ellipsometer of the present invention.

FIG. 2 is a side cross-sectional view of the reflective lens used withthe present invention.

FIG. 3 is a plan view of an alternate embodiment of the light source forthe calibration ellipsometer of the present invention.

FIG. 4 is a plan view of the composite optical measurement system withmultiple compensators in the calibration ellipsometer of the presentinvention.

FIG. 5 is an illustration of a multi-layer stack on a sample.

FIG. 6 is a flow chart illustrating the steps which can be carried outto characterize individual layers of a multi-layer stack usingmeasurements from both a stable wavelength ellipsometer and amulti-wavelength measurement.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is a composite thin film optical measurementsystem 1 having a wavelength stable reference ellipsometer 2 that isused, in conjunction with a reference sample 4 having a substrate 6 andthin film 8 with known compositions, to calibrate non-contact opticalmeasurement devices contained in the composite thin film opticalmeasurement system 1.

FIG. 1 illustrates the composite optical measurement system 1 that hasbeen developed by the present assignees, which includes five differentnon-contact optical measurement devices and the reference ellipsometer 2of the present invention.

Composite optical measurement system 1 includes a Beam ProfileEllipsometer (BPE) 10, a Beam Profile Reflectometer (BPR) 12, aBroadband Reflective Spectrometer (BRS) 14, a Deep Ultra VioletReflective Spectrometer (DUV) 16, and a Broadband SpectroscopicEllipsometer (BSE) 18. These five optical measurement devices utilize asfew as two optical sources: laser 20 and white light source 22. Laser 20generates a probe beam 24, and white light source 22 generates probebeam 26 (which is collimated by lens 28 and directed along the same pathas probe beam 24 by mirror 29). Laser 20 ideally is a solid state laserdiode from Toshiba Corp. which emits a linearly polarized 3 mW beam at673 nm. White light source 22 is ideally a deuterium-tungsten lamp thatproduces a 200 mW polychromatic beam that covers a spectrum of 200 nm to800 nm. The probe beams 24/26 are reflected by mirror 30, and passthrough mirror 42 to sample 4.

The probe beams 24/26 are focused onto the surface of the sample with alens 32 or lens 33. In the preferred embodiment, two lenses 32/33 aremounted in a turret (not shown) and are alternatively movable into thepath of probe beams 24/26. Lens 32 is a spherical, microscope objectivelens with a high numerical aperture (on the order of 0.90 NA) to createa large spread of angles of incidence with respect to the samplesurface, and to create a spot size of about one micron in diameter. Lens33 is illustrated in FIG. 2, and is a reflective lens having a lowernumerical aperture (on the order of 0.4 NA) and capable of focusing deepUV light to a spot size of about 10-15 microns.

Beam profile ellipsometry (BPE) is discussed in U.S. Pat. No. 5,181,080,issued Jan. 19, 1993, which is commonly owned by the present assigneeand is incorporated herein by reference. BPE 10 includes a quarter waveplate 34, polarizer 36, lens 38 and a quad detector 40. In operation,linearly polarized probe beam 24 is focused onto sample 4 by lens 32.Light reflected from the sample surface passes up through lens 32,through mirrors 42, 30 and 44, and directed into BPE 10 by mirror 46.The position of the rays within the reflected probe beam correspond tospecific angles of incidence with respect to the sample's surface.Quarter-wave plate 34 retards the phase of one of the polarizationstates of the beam by 90 degrees. Linear polarizer 36 causes the twopolarization states of the beam to interfere with each other. Formaximum signal, the axis of the polarizer 36 should be oriented at anangle of 45 degrees with respect to the fast and slow axis of thequarter-wave plate 34. Detector 40 is a quad-cell detector with fourradially disposed quadrants that each intercept one quarter of the probebeam and generate a separate output signal proportional to the power ofthe portion of the probe beam striking that quadrant. The output signalsfrom each quadrant are sent to a processor 48. As discussed in the U.S.Pat. No. 5,181,080 patent, by monitoring the change in the polarizationstate of the beam, ellipsometric information, such as ψ and Δ, can bedetermined. To determine this information, the processor 48 takes thedifference between the sums of the output signals of diametricallyopposed quadrants, a value which varies linearly with film thickness forvery thin films.

Beam profile reflectometry (BPR) is discussed in U.S. Pat. No.4,999,014, issued on Mar. 12, 1991, which is commonly owned by thepresent assignee and is incorporated herein by reference. BPR 12includes a lens 50, beam splitter 52 and two linear detector arrays 54and 56 to measure the reflectance of the sample. In operation, linearlypolarized probe beam 24 is focused onto sample 4 by lens 32, withvarious rays within the beam striking the sample surface at a range ofangles of incidence. Light reflected from the sample surface passes upthrough lens 32, through mirrors 42 and 30, and directed into BPR 12 bymirror 44. The position of the rays within the reflected probe beamcorrespond to specific angles of incidence with respect to the sample'ssurface. Lens 50 spatially spreads the beam two-dimensionally. Beamsplitter 52 separates the S and P components of the beam, and detectorarrays 54 and 56 are oriented orthogonal to each other to isolateinformation about S and P polarized light. The higher angles ofincidence rays will fall closer to the opposed ends of the arrays. Theoutput from each element in the diode arrays will correspond todifferent angles of incidence. Detector arrays 54/56 measure theintensity across the reflected probe beam as a function of the angle ofincidence with respect to the sample surface. The processor 48 receivesthe output of the detector arrays 54/56, and derives the thickness andrefractive index of the thin film layer 8 based on these angulardependent intensity measurements by utilizing various types of modelingalgorithms. Optimization routines which use iterative processes such asleast square fitting routines are typically employed. One example ofthis type of optimization routine is described in “MultiparameterMeasurements of Thin Films Using Beam-Profile Reflectivity,” Fanton, et.al., Journal of Applied Physics, Vol. 73, No. 11, p.7035, 1993. Anotherexample appears in “Simultaneous Measurement of Six Layers in a Siliconon Insulator Film Stack Using Spectrophotometry and Beam ProfileReflectometry, ” Leng, et. al., Journal of Applied Physics, Vol. 81, No.8, page 3570, 1997.

Broadband reflective spectrometer (BRS) 14 simultaneously probes thesample 4 with multiple wavelengths of light. BRS 14 uses lens 32 andincludes a broadband spectrometer 58 which can be of any type commonlyknown and used in the prior art. The spectrometer 58 shown in FIG. 1includes a lens 60, aperture 62, dispersive element 64 and detectorarray 66. During operation, probe beam 26 from white light source 22 isfocused onto sample 4 by lens 32. Light reflected from the surface ofthe sample passes up through lens 32, and is directed by mirror 42(through mirror 84) to spectrometer 58. The lens 60 focuses the probebeam through aperture 62, which defines a spot in the field of view onthe sample surface to analyze. Dispersive element 64, such as adiffraction grating, prism or holographic plate, angularly disperses thebeam as a function of wavelength to individual detector elementscontained in the detector array 66. The different detector elementsmeasure the optical intensities of the different wavelengths of lightcontained in the probe beam, preferably simultaneously. Alternately,detector 66 can be a CCD camera, or a photomultiplier with suitablydispersive or otherwise wavelength selective optics. It should be notedthat a monochrometer could be used to measure the different wavelengthsserially (one wavelength at a time) using a single detector element.Further, dispersive element 64 can also be configured to disperse thelight as a function of wavelength in one direction, and as a function ofthe angle of incidence with respect to the sample surface in anorthogonal direction, so that simultaneous measurements as a function ofboth wavelength and angle of incidence are possible. Processor 48processes the intensity information measured by the detector array 66.

Deep ultra violet reflective spectrometry (DUV) simultaneously probesthe sample with multiple wavelengths of ultra-violet light. DUV 16 usesthe same spectrometer 58 to analyze probe beam 26 as BRS 14, except thatDUV 16 uses the reflective lens 33 (FIG. 2) instead of focusing lens 32.To operate DUV 16, the turret containing lenses 32/33 is rotated so thatreflective lens 33 is aligned in probe beam 26. The reflective lens 33is necessary because solid objective lenses cannot sufficiently focusthe UV light onto the sample.

Broadband spectroscopic ellipsometry (BSE) is discussed in pending U.S.patent application Ser. No. 08/685,606, filed on Jul. 24, 1996, which iscommonly owned by the present assignee and is incorporated herein byreference. BSE (18) includes a polarizer 70, focusing mirror 72,collimating mirror 74, rotating compensator 76, and analyzer 80. Inoperation, mirror 82 directs at least part of probe beam 26 to polarizer70, which creates a known polarization state for the probe beam,preferably a linear polarization. Mirror 72 focuses the beam onto thesample surface at an oblique angle, ideally on the order of 70 degreesto the normal of the sample surface. Based upon well known ellipsometricprinciples, the reflected beam will generally have a mixed linear andcircular polarization state after interacting with the sample, basedupon the composition and thickness of the sample's film 8 and substrate6. The reflected beam is collimated by mirror 74, which directs the beamto the rotating compensator 76. Compensator 76 introduces a relativephase delay δ (phase retardation) between a pair of mutually orthogonalpolarized optical beam components. Compensator 76 is rotated at anangular velocity Ω about an axis substantially parallel to thepropagation direction of the beam, preferably by an electric motor 78.Analyzer 80, preferably another linear polarizer, mixes the polarizationstates incident on it. By measuring the light transmitted by analyzer80, the polarization state of the reflected probe beam can bedetermined. Mirror 84 directs the beam to spectrometer 58, whichsimultaneously measures the intensities of the different wavelengths oflight in the reflected probe beam that pass through thecompensator/analyzer combination. Processor 48 receives the output ofthe detector 66, and processes the intensity information measured by thedetector 66 as a function of wavelength and as a function of the azimuth(rotational) angle of the compensator 76 about its axis of rotation, tosolve the ellipsometric values Ψ, and Δ as described in U.S. patentapplication Ser. No. 08/685,606.

Detector/camera 86 is positioned above mirror 46, and can be used toview reflected beams off of the sample 4 for alignment and focuspurposes.

In order to calibrate BPE 10, BPR 12, BRS 14, DUV 16, and BSE 18, thecomposite optical measurement system 1 includes the wavelength stablecalibration reference ellipsometer 2 used in conjunction with areference sample 4. Ellipsometer 2 includes a light source 90, polarizer92, lenses 94 and 96, rotating compensator 98, analyzer 102 and detector104.

Light source 90 produces a quasi-monochromatic probe beam 106 having aknown stable wavelength and stable intensity. This can be donepassively, where light source 90 generates a very stable outputwavelength which does not vary over time (i.e. varies less than 1%).Examples of passively stable light sources are a helium-neon laser, orother gas discharge laser systems. Alternately, a non-passive system canbe used as illustrated in FIG. 3 where the light source 90 includes alight generator 91 that produces light having a wavelength that is notprecisely known or stable over time, and a monochrometer 93 thatprecisely measures the wavelength of light produced by light generator91. Examples of such light generators include solid state lasers, laserdiodes, or polychromatic light sources used in conjunction with a colorfilter such as a grating. In either case, the wavelength of beam 106,which is a known constant or measured by monochrometer 93, is providedto processor 48 so that ellipsometer 2 can accurately calibrate theoptical measurement devices in system 1.

The beam 106 interacts with polarizer 92 to create a known polarizationstate. In the preferred embodiment, polarizer 92 is a linear polarizermade from a quartz Rochon prism, but in general the polarization doesnot necessarily have to be linear, nor even complete. Polarizer 92 canalso be made from calcite. The azimuth angle of polarizer 92 is orientedso that the plane of the electric vector associated with the linearlypolarized beam exiting from the polarizer 92 is at a known angle withrespect to the plane of incidence (defined by the propagation directionof the beam 106 and the normal to the surface of sample 4). The azimuthangle is preferably selected to be on the order of 30 degrees becausethe sensitivity is optimized when the reflected intensities of the P andS polarized components are approximately balanced. It should be notedthat polarizer 92 can be omitted if the light source 90 emits light withthe desired known polarization state.

The beam 106 is focused onto the sample 4 by lens 94 at an obliqueangle. For calibration purposes, reference sample 4 ideally consists ofa thin oxide layer 8 having a thickness d, formed on a silicon substrate6. However, in general, the sample 4 can be any appropriate substrate ofknown composition, including a bare silicon wafer, and silicon wafersubstrates having one or more thin films thereon. The thickness d of thelayer 8 need not be known, or be consistent between periodiccalibrations. The useful light from probe beam 106 is the lightreflected by the sample 4 symmetrically to the incident beam about thenormal to the sample surface. It is noted however that the polarizationstate of nonspecularly scattered radiation can be determined by themethod of the present invention as well. The beam 106 is ideallyincident on sample 4 at an angle on the order of 70 degrees to thenormal of the sample surface because sensitivity to sample properties ismaximized in the vicinity of the Brewster or pseudo-Brewster angle of amaterial. Based upon well known ellipsometric principles, the reflectedbeam will generally have a mixed linear and circular polarization stateafter interacting with the sample, as compared to the linearpolarization state of the incoming beam. Lens 96 collimates beam 106after its reflection off of the sample 4.

The beam 106 then passes through the rotating compensator (retarder) 98,which introduces a relative phase delay 6 (phase retardation) between apair of mutually orthogonal polarized optical beam components. Theamount of phase retardation is a function of the wavelength, thedispersion characteristics of the material used to form the compensator,and the thickness of the compensator. Compensator 98 is rotated at anangular velocity ω about an axis substantially parallel to thepropagation direction of beam 106, preferably by an electric motor 100.Compensator 98 can be any conventional wave-plate compensator, forexample those made of crystal quartz. The thickness and material of thecompensator 98 are selected such that a desired phase retardation of thebeam is induced. In the preferred embodiment, compensator 98 is abi-plate compensator constructed of two parallel plates of anisotropic(usually birefringent) material, such as quartz crystals of oppositehandedness, where the fast axes of the two plates are perpendicular toeach other and the thicknesses are nearly equal, differing only byenough to realize a net first-order retardation for the wavelengthproduced by the light source 90.

Beam 106 then interacts with analyzer 102, which serves to mix thepolarization states incident on it. In this embodiment, analyzer 102 isanother linear polarizer, preferably oriented at an azimuth angle of 45degrees relative to the plane of incidence. However, any optical devicethat serves to appropriately mix the incoming polarization states can beused as an analyzer. The analyzer 102 is preferably a quartz Rochon orWollaston prism. The rotating compensator 98 changes the polarizationstate of the beam as it rotates such that the light transmitted byanalyzer 102 is characterized by: $\begin{matrix}{{I(t)} = {\left( {1/2} \right)\left\lbrack {{{E_{x}}^{2}\left( {1 + {\cos^{2}\left( {\delta/2} \right)} + {{E_{y}}^{2}{\sin^{2}\left( {\delta/2} \right)}}} \right\rbrack}\quad - {{{Im}\left( {E_{x}E_{y}^{*}} \right)}\sin \quad {{\delta sin}\left( {2\omega \quad t} \right)}}\quad + {{{Re}\left( {E_{x}E_{y}^{*}} \right)}{\sin \quad}^{2}\left( {\delta/2} \right){\sin \left( {4\omega \quad t} \right)}}\quad + {\left( {1/2} \right)\left( {{E_{x}}^{2} - {E_{y}}^{2}} \right){\sin^{2}\left( {\delta/2} \right)}{\cos \left( {4\omega \quad t} \right)}}} \right.}} & (1) \\{\quad {{= {a_{o} + {b_{2}{\sin \left( {2\omega \quad t} \right)}} + {a_{4}{\cos \left( {4\omega \quad t} \right)}} + {b_{2}{\sin \left( {4\omega \quad t} \right)}}}},}} & (2)\end{matrix}$

where E_(x) and E_(y) are the projections of the incident electric fieldvector parallel and perpendicular, respectively, to the transmissionaxis of the analyzer, δ is the phase retardation of the compensator, andω is the angular rotational frequency of the compensator.

For linearly polarized light reflected at non-normal incidence from thespecular sample, we have

E _(x) =r _(p)cosP  (3a)

E _(y) =r _(s)sinP  (3b)

where P is the azimuth angle of the incident light with respect to theplane of incidence. The coefficients a₀, b₂, a₄, and b₄ can be combinedin various ways to determine the complex reflectance ratio:

r _(p) /r _(s)=tanψe ^(iΔ).  (4)

It should be noted that the compensator 98 can be located either betweenthe sample 4 and the analyzer 102 (as shown in FIG. 1), or between thesample 4 and the polarizer 92, with appropriate and well known minorchanges to the equations. It should also be noted that polarizer 70,lenses 94/96, compensator 98 and polarizer 102 are all optimized intheir construction for the specific wavelength of light produced bylight source 90, which maximizes the accuracy of ellipsometer 2.

Beam 106 then enters detector 104, which measures the intensity of thebeam passing through the compensator/analyzer combination. The processor48 processes the intensity information measured by the detector 104 todetermine the polarization state of the light after interacting with theanalyzer, and therefore the ellipsometric parameters of the sample. Thisinformation processing includes measuring beam intensity as a functionof the azimuth (rotational) angle of the compensator about its axis ofrotation. This measurement of intensity as a function of compensatorrotational angle is effectively a measurement of the intensity of beam106 as a function of time, since the compensator angular velocity isusually known and a constant.

By knowing the composition of reference sample 4, and by knowing theexact wavelength of light generated by light source 90, the opticalproperties of reference sample 4, such as film thickness d, refractiveindex and extinction coefficients, etc., can be determined byellipsometer 2. If the film is very thin, such as less than or equal toabout 20 angstroms, the thickness d can be found to first order in d/λby solving $\begin{matrix}{{\frac{\rho - \rho_{o}}{\rho_{o}} = {\frac{4\quad \pi \quad {id}\quad \cos \quad \theta}{\lambda}\quad \frac{{\varepsilon_{s}\left( {\varepsilon_{s} - \varepsilon_{o}} \right)}\left( {\varepsilon_{o} - \varepsilon_{a}} \right)}{{\varepsilon_{o}\left( {\varepsilon_{s} - \varepsilon_{a}} \right)}\quad \left( {{\varepsilon_{s}\cot^{2}\theta} - \varepsilon_{a}} \right)}}},} & (5)\end{matrix}$

where

ρ_(o)=tanψ_(o) e ^(iΔo)  (6)

$\begin{matrix}{{= \quad}\frac{{\sin^{2}\quad \theta}\quad - \quad {\cos \quad {\theta \left( {{\varepsilon_{s}/\varepsilon_{a}}\quad - \quad {\sin^{2}\quad \theta}} \right)}^{1/2}}}{{\sin^{2}\quad \theta}\quad + \quad {\cos \quad {\theta \left( {{\varepsilon_{s}/\varepsilon_{a}}\quad - \quad {\sin^{2}\quad \theta}} \right)}^{1/2}}}} & (7)\end{matrix}$

which is the value of ρ=tanψe^(iΔ) for d=0. Here, λ=wavelength of light;and ε_(s), ε_(o) and ε_(a) are the dielectric functions of thesubstrate, thin oxide film, and ambient, respectively, and θ is theangle of incidence.

If the film thickness d is not small, then it can be obtained by solvingthe equations

ρ=r _(p) /r _(s), where  (8) $\begin{matrix}{r_{p}\quad = \quad \frac{r_{p,\quad {oa}}\quad + \quad {Zr}_{p,\quad {so}}}{1\quad + \quad {{Zr}_{p,\quad {oa}}\quad r_{p,\quad {so}}}}} & (9) \\{r_{s}\quad = \quad \frac{r_{s,\quad {oa}}\quad + \quad {Zr}_{s,\quad {so}}}{1\quad + \quad {{Zr}_{s,\quad {oa}}\quad r_{s,\quad {so}}}}} & (10)\end{matrix}$

and where

Z=e^(2ik d),  (11)

ck _(o⊥) /ω=n _(o⊥)=(ε_(o) /ε _(a)−sin²θ)^(½)  (12)

$\begin{matrix}{r_{s,\quad {so}}\quad = \quad \frac{n_{o\bot}\quad - \quad n_{s\bot}}{n_{o\bot}\quad + \quad n_{s\bot}}} & (13) \\{r_{s,\quad {oa}}\quad = \quad \frac{n_{a\bot}\quad - \quad n_{o\bot}}{n_{a\bot}\quad + \quad n_{o\bot}}} & (14) \\{r_{p,\quad {so}}\quad = \quad \frac{{\varepsilon_{s}\quad n_{o\bot}}\quad - \quad {\varepsilon_{o}\quad n_{s\bot}}}{{{\varepsilon \quad}_{s}\quad n_{o\bot}}\quad + \quad {\varepsilon_{o}\quad n_{s\bot}}}} & (15) \\{r_{p,\quad {oa}}\quad = \quad \frac{{\varepsilon_{o}\quad n_{a\bot}}\quad - \quad {\varepsilon_{a}\quad n_{o\bot}}}{{{\varepsilon \quad}_{o}\quad n_{a\bot}}\quad + \quad {\varepsilon_{a}\quad n_{o\bot}}}} & (16)\end{matrix}$

and in general

n _(j⊥)=(ε_(j)−ε_(a)sin²θ)^(½)  (17)

where j is s or a. These equations generally have to be solvednumerically for d and n_(o) simultaneously, using ε_(s), ε_(a), λ, andθ, which are known.

Once the thickness d of film 8 has been determined by ellipsometer 2,then the same sample 4 is probed by the other optical measurementdevices BPE 10, BPR 12, BRS 14, DUV 16, and BSE 18 which measure variousoptical parameters of the sample 4. Processor 48 then calibrates theprocessing variables used to analyze the results from these opticalmeasurement devices so that they produce accurate results. For each ofthese measurement devices, there are system variables that affect themeasured data and need to be accounted for before an accuratemeasurement of other samples can be made. In the case of BPE 10, themost significant variable system parameter is the phase shift thatoccurs due to the optical elements along the BPE optical path.Environmental changes to these optical elements result in an overalldrift in the ellipsometric parameter Δ, which then translates into asample thickness drift calculated by the processor 48 from BPE 10. Usingthe measured optical parameters of BPE 10 on reference sample 4, andusing Equation 5 and the thickness of film 8 as determined fromcalibration ellipsometer 2, the processor 48 calibrates BPE 10 byderiving a phase offset which is applied to measured results from BPE 10for other samples, thereby establishing an accurate BPE measurement. ForBSE 18, multiple phase offsets are derived for multiple wavelengths inhe measured spectrum.

For the remaining measurement devices, BPR 12, BRS 14 and DUV 16, themeasured reflectances can also be affected by environmental changes tothe optical elements in the beam paths. Therefore, the reflectancesR_(ref) measured by BPR 12, BRS 14 and DUV 16 for the reference sample 4are used, in combination with the measurements by ellipsometer 2, tocalibrate these systems. Equations 9-17 are used to calculate theabsolute reflectances R^(c) _(ref) of reference sample 4 from themeasured results of ellipsometer 2. All measurements by the BPR/BRS/DUVdevices of reflectance (R_(s)) for any other sample are then scaled byprocessor 48 using the normalizing factor in equation 18 below to resultin accurate reflectances R derived from the BPR, BRS and DUV devices:

R=R _(s)(R ^(c) _(ref) /R _(ref))  (18)

In the above described calibration techniques, all system variablesaffecting phase and intensity are determined and compensated for usingthe phase offset and reflectance normalizing factor discussed above,thus rendering the optical measurements made by these calibrated opticalmeasurement devices absolute.

The above described calibration techniques are based largely uponcalibration using the derived thickness d of the thin film. However,calibration using ellipsometer 2 can be based upon any of the opticalproperties of the reference sample that are measurable or determinableby ellipsometer 2 and/or are otherwise known, whether the sample has asingle film thereon, has multiple films thereon, or even has no filmthereon (bare sample).

The advantage of the present invention is that a reference sample havingno thin film thereon, or having thin film thereon with an unknownthickness which may even vary slowly over time, can be repeatedly usedto accurately calibrate ultra-sensitive optical measurement devices.

The output of light source 90 can also be used to calibrate thewavelength measurements made by spectrometer 58. The sample 4 can betipped, or replaced by a tipped mirror, to direct beam 106 up to mirror42 and to dispersion element 64. By knowing the exact wavelength oflight produced by light source 90, processor 48 can calibrate the outputof detector 66 by determining which pixel(s) corresponds to thatwavelength of light.

It should be noted that the calibrating ellipsometer 2 of the presentinvention is not limited to the specific rotating compensatorellipsometer configuration discussed above. The scope of the presentinvention includes any ellipsometer configuration in conjunction withthe light source 90 (having a known wavelength) that measures thepolarization state of the beam after interaction with the sample andprovides the necessary information about sample 4 for calibratingnon-contact optical measurement devices.

For example, another ellipsometric configuration is to rotate polarizer92 or analyzer 100 with motor 100, instead of rotating the compensator98. The above calculations for solving for thickness d still apply.

In addition, null ellipsometry, which uses the same elements asellipsometer 2 of FIG. 1, can be used to determine the film thickness dfor calibration purposes. The ellipsometric information is derived byaligning the azimuthal angles of these elements until a null or minimumlevel intensity is measured by the detector 104. In the preferred nullellipsometry embodiment, polarizers 92 and 102 are linear polarizers,and compensator 98 is a quarter-wave plate. Compensator 98 is aligned sothat its fast axis is at an azimuthal angle of 45 degrees relative tothe plane of incidence of the sample 4. Polarizer 92 has a transmissionaxis that forms an azimuthal angle P relative to the plane of incidence,and polarizer 102 has a transmission axis that forms an azimuthal angleA relative to the plane of incidence. Polarizers 92 and 102 are rotatedabout beam 106 such that the light is completely extinguished(minimized) by the analyzer 102. In general, there are two polarizer92/102 orientations (P₁, A₁) and (P₂, A₂) that satisfy this conditionand extinguish the light. With the compensator inducing a 90 degreephase shift and oriented with an azimuthal angle at 45 degree relativeto the plane of incidence, we have:

P ₂ =P ₁±π  (19)

A ₂ =−A ₁  (20)

ψ=A ₁>0  (21)

(where A₁ is the condition for which A is positive).

Δ=2P ₁+π/2  (22)

which, when combined with equations 5-10, allows the processor to solvefor thickness d.

Null ellipsometry is very accurate because the results depend entirelyon the measurement of mechanical angles, and are independent ofintensity. Null ellipsometry is further discussed by R. M. A. Azzam andN. M. Bashara, in Ellipsometry and Polarized Light (North-Holland,Amsterdam, 1977); and by D. E. Aspnes, in Optical Properties of Solids:New Developments, ed. B. O. Seraphin (North-Holland, Amsterdam, 1976),p. 799.

It is also conceivable to omit compensator 98 from ellipsometer 2, anduse motor 100 to rotate polarizer 92 or analyzer 102. Either thepolarizer 92 or the analyzer 102 is rotated so that the detector signalcan be used to accurately measure the linear polarization component ofthe reflected beam. Then, the circularly polarized component is inferredby assuming that the beam is totally polarized, and what is not linearlypolarized must be circularly polarized. Such an ellipsometer, commonlycalled a rotating-polarizer or rotating-analyzer ellipsometer, is termed“an incomplete” polarimeter, because it is insensitive to the handednessof the circularly polarized component and exhibits poor performance whenthe light being analyzed is either nearly completely linearly polarizedor possesses a depolarized component. However, using UV light fromsource 90, the substrate of materials such as silicon contribute enoughto the overall phase shift of the light interacting with the sample thataccurate results can be obtained without the use of a compensator. Insuch a case, the same formulas above can be used to derive thickness d,where the phase shift induced by the compensator is set to be zero.

It is to be understood that the present invention is not limited to theembodiments described above and illustrated herein, but encompasses anyand all variations falling within the scope of the appended claims. Forexample, beams 24, 26, and/or 106 can be transmitted through the sample,where the beam properties (including the beam polarization state) of thetransmitted beam are measured. Further, a second compensator can beadded, where the first compensator is located between the sample and theanalyzer, and the second compensator located between the sample and thelight source 90, as illustrated in FIG. 4. These compensators could bestatic or rotating. In addition, to provide a static or varyingretardation between the polarization states, compensator 98 can bereplaced by a non-rotating opto-electronic element or photo-elasticelement, such as a piezo-electric cell retarder which are commonly usedin the art to induce a sinusoidal or static phase retardation byapplying a varying or static voltage to the cell.

After the apparatus has been calibrated, it can be used to make avariety of measurements. One type of measurement of significant interestto the semiconductor industry is the characterization of multi-layerthin films on a substrate. FIG. 5 is an illustration of such a sample200. Sample 200 includes a semiconductor substrate 202 which istypically silicon but could be germanium, gallium arsenide, etc. Aplurality of thin film layers are deposited on top of the substrate. Thethickness of these layers in the illustration has been exaggerated forclarity.

As seen in the example of FIG. 5, four thin film layers 204 to 210 aredeposited on the stack. The most typical materials used to form thinfilm layers include oxides, nitrides, polysilicon, titanium andtitanium-nitride. Each of these materials have different opticalcharacteristics. As the number and variation of the thin film layersincreases, it becomes increasingly difficult to determinecharacteristics of individual layers even if multiple measurements aretaken.

In accordance with the subject invention, the reference ellipsometer ofthe subject system can further be used to help better analyze complexmulti-layer stacks. Although the output from the reference ellipsometeris limited and is not particularly helpful in analyzing individuallayers in a stack, it can be used to provide a very accuratedetermination of the total optical thickness T of the stack. Asdiscussed below, the processor 48 can use the measurements obtained fromthe reference ellipsometer in combination with the other measurements toimprove the accuracy of the analysis of the individual layers.

FIG. 6 is a flow chart illustrating how the system can be configured toanalyze multi-layer stacks. The steps shown in FIG. 6 would generallyoccur after calibration in the manner discussed above. In addition, itshould be noted that the data gathering steps are shown in sequentialorder in FIG. 6 for illustration purposes only. In fact, the variousmeasurements can be made in any order. The results are stored in theprocessor as each measurement is completed. When all the desiredmeasurements are completed, then the processor can analyze the data.

In accordance with the subject invention, the ellipsometer 2 is used tomeasure the test sample (step 230). In this case, the test sample 200would be placed in the apparatus in place of the reference sample 4shown in FIG. 1. The output from the measurement, in the form of firstoutput signals, would be sent to the processor 48 in step 232.

As noted above, the output of the ellipsometer 2 will be used tocalculate the total optical thickness T of the layers. To the extentthat the ellipsometer is used for this purpose, it is preferable thatthe light source 90 be a laser which generates a fixed and knownwavelength. In the preferred embodiment, light source 90 is a heliumneon laser having a fixed output of 632.8 nanometers. The advantage ofthe helium neon laser is that it is low in cost, can be tightly focusedand generates a known wavelength output regardless of room temperatureor power levels.

In accordance with the subject invention, additional measurements mustbe taken in order to analyze the characteristics of individual layers.In the preferred embodiment, the most desirable measurement will includea multi-wavelength measurement as shown in step 234. Thismulti-wavelength measurement may be based on either the change in phaseor magnitude of a reflected beam. As noted above, the white light source22 can be used for either type of measurement. The detector 58 canmeasure changes in magnitude of the reflected beam across a largewavelength range for either the broadband reflective spectrometer (BRS)14 or the deep ultraviolet reflective spectrometer (DUV) 16. Thedetector 58 generates output signals corresponding to a plurality ofwavelengths. Step 236 indicates the spectroscopic magnitude measurement.

Changes in phase of the beam at multiple wavelengths can be obtainedfrom the broadband spectroscopic ellipsometer (BSE) 18. Step 238illustrates the BSE measurement. The second output signals correspondingto the different wavelengths of either type of multi-wavelengthmeasurement are sent to the processor 48 for storage (step 240). In thepreferred embodiment, both magnitude and phase measurements are takenand sent to the processor.

Additional measurements are desirable to help more accuratelycharacterize the layers. In the preferred embodiment, these measurementsinclude those taken by the beam profile ellipsometer system (BPE) instep 242 and beam profile reflectometer system (BPR) in step 244. Theresults from these measurements are sent to the processor in step 246.

In accordance with the subject invention, the processor can use thecombination of inputs from the measurement systems to characterize thesample. As noted above, the processor will typically include a modelingalgorithm which utilizes an iterative process such as a least squaresfitting routine to determine the characteristics of individual layers.(See, for example, the Fanton, et. al. and Leng et. al. articles, citedabove). In these types of routines, an initial calculation of theparameters of the stack is made using Fresnel equations and apredetermined “best guess” of layer characteristics. The calculationproduces a set of theoretical values which correspond to a set ofmeasurement results that can be obtained using the various test systemsin the device. The set of theoretical values are then compared to theset of measurements that were actually obtained from the various testsystems and an evaluation is made as to the closeness or “fit” betweenthe actual and theoretical values. A new “best guess” is then made as tothe layer characteristics based on how much and in what manner thetheoretical values differed from the measured values. The algorithmrecalculates the parameters once again using the Fresnel equations andanother comparison is made between the revised theoretical values andthe experimentally obtained measurements. This process is continued inan iterative fashion until the theoretical values match the actualmeasured values to a predetermined level of accuracy.

In accordance with the subject invention, this mathematical modeling isexpanded to include parameters representative of the total opticalthickness of the stack (step 250). This analysis assumes that themultilayers stack is actually a single layer with commoncharacteristics. The model will generate a set of additional theoreticalvalues corresponding to the measurements which should be generated bythe narrow-band, off-axis ellipsometer measurement. During the iterativeprocess, these theoretical values associated with the total opticalthickness are compared with actual measured values obtained from theoff-axis ellipsometer. The closeness of the “fit” between all of thetheoretical values (including the values associated with the totaloptical thickness) and all of the measured values is evaluated in theiterative process to generate a more accurate analysis of thecharacteristics of the individual layers in the stack.

The improvements achieved by this approach are derived from the factthat the total optical thickness of a stack can be very accuratelydetermined from measurements using an off-axis ellipsometer with a knownwavelength. For stacks ranging up to 200 angstroms thick, this type ofmeasurement can be accurate to within a single angstrom or less.

The subject invention is not limited to the particular algorithm used toderive the characteristics of the individual layers. In addition to themore conventional least square fitting routines, alternative approachescan be used. For example, the high level of computing power nowavailable permits approaches to be utilized which include geneticalgorithms. One example of the use of genetic algorithms to determinethe thickness of thin film layers can be found in “Using GeneticAlgorithms with Local Search for Thin Film Metrology,” Land, et. al.,Proceeding of the Seventh International Conference on GeneticAlgorithms, July 19-23, page 537, 1997. The only requirement of thesubject invention is that the algorithm be designed such that themeasurements from the off-axis ellipsometer be used to evaluate thetheoretical overall optical thickness of the multilayer stack and thatthis information be used to help minimize ambiguities in the analysis ofthe characteristics of the individual layers.

While the subject invention has been described with reference to apreferred embodiment, various changes and modifications could be madetherein, by one skilled in the art, without varying from the scope andspirit of the subject invention as defined by the appended claims.

What is claimed is:
 1. A method for analyzing a sample having a multiplelayer thin film stack thereon comprising the steps of generating a firstprobe beam from a laser having a narrowband output which is stable towithin one percent; directing the first probe beam to reflect off thesurface of the sample at a non-normal angle of incidence; analyzing thechange in polarization state of the first probe beam induced by theinteraction with the sample and generating first output signals inresponse thereto; generating a second probe beam from a broad bandwavelength source; directing said second probe beam to reflect off thesurface of the sample; monitoring the second probe beam after reflectionfrom the sample and determining the change in polarization state at aplurality of wavelengths and simultaneously generating a plurality ofsecond output signals corresponding thereto; and analyzing thecharacteristics of the thin film layers on the sample based on acombination of the first and second output signals.
 2. A method asrecited in claim 1, wherein the first probe beam is generated by a gasdischarge laser.
 3. A method as recited in claim 2, wherein a rotatingcompensator is located in the path of the first probe beam.
 4. A methodas recited in claim 1, wherein the broad band light source generateslight having a wavelength range between at least 200 and 800nm.
 5. Amethod as recited in claim 1, wherein the first output signals are usedto provide an accurate measure of the overall optical thickness of thestack in order to improve the accuracy of the analysis of the individuallayers.
 6. A method as recited in claim 1 further including the step ofmonitoring the change in magnitude of the second probe beam induced byreflection off the surface of the sample at a plurality of wavelengthsand generating third output signals and wherein the characteristics ofthe thin film layers on the sample are analyzed based on a combinationof the first, second and third output signals.
 7. A method as recited inclaim 1, wherein the second probe beam is directed to reflect off thespot on the sample at a non-normal angle of incidence.
 8. An apparatusfor analyzing a sample having a multiple layer thin film stack thereoncomprising: an off-axis ellipsometer, said ellipsometer including alaser for generating a narrowband output which is stable to within 1percent, said output defining a first probe beam, said ellipsometer formeasuring the change in polarization state of the first probe beam afterreflection from the sample and generating first output signalscorresponding thereto; a broad band light source for generating a secondprobe beam; a detector system for analyzing the change in polarizationstate of the second probe beam after interacting with the sample andsimultaneously generating a plurality of second output signalscorresponding to a plurality of different wavelengths; and a processorfor analyzing the characteristics of the thin film layers on the samplebased on a combination of the first and second output signals.
 9. Anapparatus as recited in claim 8, wherein the laser of said firstellipsometer is a gas discharge laser.
 10. An apparatus as recited inclaim 9, wherein said first ellipsometer includes a rotatingcompensator.
 11. An apparatus as recited in claim 8, wherein the broadband light source generates light having a wavelength range between atleast 200 and 800nm.
 12. An apparatus as recited in claim 8, wherein theprocessor uses an algorithm wherein the first output signals are used toprovide an accurate measure of the overall optical thickness of thestack in order to improve the accuracy of the analysis of the individuallayers.
 13. An apparatus as recited in claim 8 wherein the detectorsystem also monitors the change in magnitude of the second probe beaminduced by reflection off the surface of the sample at a plurality ofwavelengths and generates third output signals and wherein the processoranalyzes the characteristics of the thin film layers on the sample basedon a combination of the first, second and third output signals.
 14. Anapparatus as recited in claim 8, wherein the second probe beam isdirected to reflect off the spot on the sample at a non-normal angle ofincidence.